1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices. More specifically, the present invention relates to a method of manufacturing semiconductor devices, wherein cracking can be prevented from occurring in an insulating film in a metal layer metal (hereinafter, referred to as “MLM”) process.
2. Discussion of Related Art
A MLM formation method of semiconductor devices includes forming metal lines, such as aluminum (Al) and tungsten (W), and providing insulation among the metal lines using a silicon oxide film such as high density plasma (hereinafter, referred to as “HDP”).
After such a metal process is completed, annealing for improved refresh characteristics and metal resistance stabilization has to be performed.
Generally, metal is several tens of times higher in the degree of thermal expansion than the insulating film. It is thus inevitable that thermal stress occurs due to a difference in the degree of thermal expansion between the metal and the oxide film upon annealing. Such thermal stress causes cracks to occur at weak points of the insulating film.
FIG. 1 is a scanning electron microscope (SEM) photograph showing cracking created in an insulating film due to thermal stress. In FIG. 1, a first insulating film crack is generated due to thermal stress and a second crack is formed due to depression by the first crack.
Furthermore, an annealing process is usually performed at a temperature of 400 to 450° C. for about 20 to 30 minutes. If an aluminum line is used, a melting temperature of aluminum is 600° C., which is high temperature of about ⅔ Tm, and thus causes liquid behavior. Accordingly, aluminum for lines shows liquid behavior during annealing process.
At this time, if cracks due to thermal stress exist, Al is infiltrated into the cracks due to a capillary phenomenon and Al volume expansion. This results in metal line bridge failure.
FIG. 2 is a SEM photograph showing metal line bridge failure. FIG. 2 shows that metal (Al) is infiltrated into a crack, and is thus connected to an underlying conduction layer, as indicated by “A”.
If metal and an insulating film exist, it is inevitable that thermal stress is generated due to a difference in the degree of thermal expansion between the metal and the insulating film upon annealing. In this case, the degree of stress applied to the insulating film is compression stress of approximately −3.5E11 dyn/cm2. More particularly, stress is concentrated on a metal line corner area. Often, stress which is several hundreds of times higher than ˜1E9 dyn/cm2 being a general thin film stress level, is generated.
Stress (σox) applied to an insulating film due to MLM can be expressed into the following equation.
      σ    ox    =                    [                              E            Al                                (                          1              -                              v                Al                                      )                          ]            ⋆                        (                                    α              ox                        -                          α              Al                                )                ⁢        dT              =                  -        3.5            ⁢      E      ⁢                          ⁢      11      ⁢                          ⁢      dyn      ⁢              /            ⁢              cm        2            where, σox is stress applied to an insulating film by metal,                EAl is the modulus of elasticity of Al (=6E11 dyn/cm2),        νAl is the Poisson's ratio(=0.3) of Al,        αAl is the degree of thermal expansion (=10 ppm/k) of Al,        αax is the degree of thermal expansion (=0.55 ppm/k) of insulating film, and        dT is a difference between normal temperature and temperature upon annealing (=425K)        
FIGS. 3A and 3B are graphs showing a stress hysteresis curve of Al and a HDP (High Density Plasma) oxide film depending upon temperature.
As can be seen from FIG. 3A, although aluminum has tensile stress at an initial stage, it is changed to compression stress since the degree of thermal expansion is high as the temperature rises. However, the compression stress is changed to tensile stress again upon cooling.
However, a seen in FIG. 3B, a HDP oxide film has compression stress at an initial stage. As temperature rises, the compression stress is changed to tensile stress, but the HDP oxide film still shows compression stress. The compression stress then increases again upon cooling.
FIGS. 4A to 4C are schematic diagrams illustrating the state of stress generated in Al and the HDP oxide film upon annealing based on the stress hysteresis curves of FIGS. 3A and 3B.
In the drawings, an arrow direction indicates a direction along which a file tries to be oriented, and the length of the arrow indicates the amount of stress.
Before heating (FIG. 4A) and in a cooling (FIG. 4C) state, Al shows tensile stress and a HDP oxide film shows compression stress. As the directions of their stress exert in an opposite direction, stress can be mitigated.
In a heating (FIG. 4B) state, however, since the stress of Al is changed to compression stress, both Al and the HDP oxide film show the compression stress. Thus, very high stress is generated.
Accordingly, as indicated by “B” in FIG. 4B, stress is concentrated on metal line bottom corner areas.
Al bottom corner areas undergo over etch of about 50% in order to remove metal line bridges upon formation of metal lines. In this case, the Al bottom corner areas are areas where an underlying TEOS oxide film is lost due to over etch.
FIG. 5 is a view illustrating an Al etch profile. From FIG. 5, it can be seen that a TEOS oxide film is lost due to over etch at the bottom corner areas.
This Al bottom area is formed as a heterogeneous film interface (TEOS/HDP) along with concentration of stress, and is very weak and susceptible to cracking.
FIG. 6 is a SEM photograph showing crack of an over-etch area and an Al infiltration phenomenon through crack. From FIG. 6, it can be seen that the Al bottom area is weak and cracked, and accordingly, there are problems of Al infiltration.
Furthermore, the TEOS oxide film contains a large amount of moisture, hydro-carbon impurity, etc. on its surface. The interface of the TEOS oxide film is very unstable and always has problems.
FIG. 7 is a graph showing the results of measuring impurities on the surface of a TEOS oxide film using SIMS.
From FIG. 7, it can be seen that a large amount of impurities, such as H2, CxHy, H2O, CO, O2 and CO2, is contained on the surface of the TEOS oxide film. It is thus possible to predict instability of the interface.
FIG. 8 shows the degree of TEOS/TEOS interface attack. Although only vacuum break is performed after a TEOS oxide film is deposited, the TEOS oxide film is deposited again, and only wet cleaning is performed, attack is generated in the TEOS/TEOS interface due to a high etch rate. From this, it is possible to estimate that TEOS/HDP interface adhesion is very poor.
Therefore, crack in the TEOS/HDP interface of the metal line over-etch area begins due to the aforementioned reason, and Al bridge failure occurs through the crack.